Cooperative communication is regarded as one important technique in next generation wireless communication networks. A minimal single-user cooperative communication network includes a source station (SS), a relay station (RS), and a destination station (DS). As a repeater, the RS receives signals transmitted by the SS, performs appropriate signal processing, and then relays the signals to the DS. Relay techniques can increase the coverage, decrease overall transmit power, and improve reliability of the communication links with the help of multiple independent paths under multi-rely case.
Relaying modes can be broadly categorized as decode-and-forward (DF) and amplify-and forward (AF). In the DF mode, the RS demodulates the received signals, decodes the encoded data in the signals, re-encodes and re-modulates the data, and forwards the signal to the DS. In the AF mode, the RS only amplifies and forwards the received signals without decoding. Thus, the AF mode is simpler to implementation than the DF mode, and hence provides a tradeoff between performance and complexity.
In a relay-based two-hop cooperative communication network, the SS and the RS can concurrently transmit signals using the same channel, i.e., frequency subcarrier band, while the signals are detected jointly by the DS. Alternatively, the signals are transmitted using two orthogonal channels by means of time-division or frequency-division to avoid interference. In either case, cooperative diversity can be achieved by allowing the DS to concurrently receive signals from both the SS and RS.
In a scattering environment, multi-path fading varies significantly on the scale of half a wavelength. Multi-input-multi-output (MIMO) techniques use multiple antennas at both the transmitter and receiver to take advantage of the inherent spatial diversity in wireless channels. MIMO techniques are widely used to enhance the spectrum efficiency and reliability of wireless communication networks. It is desired to combine relay and MIMO techniques.
In a two-hop AF relay MIMO network, the overall channel from the SS to the DS is a combination of two interim channels. The first interim channel is from the SS to the RS, and the second interim channel is from the RS to the DS.
Conventionally, the overall channel form the SS to the DS can only be estimated directly by using pilot signals that are transmitted from the SS to the DS. However, in a typical two-hop MIMO AF relay network with simple relay nodes, the RS does not explicitly decode and retransmit pilot signals to the DS, because the RS only amplifies and forwards. As a result, the interim channels cannot be estimated directly by the DS based on pilot signals. However, the performance of the two-hop MIMO AF relay network can be significantly improved if channel state information (CSI) would be available for the interim channels. Then, the transmitters select the optimal antennas to maximize throughput and reduce errors.
Therefore, it is desired to provide a low-complexity method that estimates the CSI for the interim channels at a DS in a two-hop MIMO AF relay network without using pilot signals.